Aldehydes are widely used in many industrial processes. The US production of acetaldehyde in 1982 was 281,000 tons. The world production of acrolein in 1975 was 59,000 tons. Aldehydes occur in nature and are gaseous by-products of incomplete combustion of wood and coal, in exhaust from gasoline and diesel engines, industrial waste gases and fumes, tobacco smoke, and wood fires. Formaldehyde and acetaldehyde are carcinogens. Many of the aldehydes are mutagens. They are reactive compounds participating in oxidation, reduction, addition, and polymerization reactions. All the aldehydes possess anesthetic properties, but this is obscured by their highly irritating action on the eyes and mucous membranes of the respiratory tract. The lower aldehydes, very soluble in water, act chiefly on the eyes and tissues of the upper respiratory tract. The higher aldehydes, less soluble in water, tend to penetrate more deeply into the respiratory system and may affect the lungs. Some higher aldehydes and also the aromatic aldehydes may exhibit much lower toxicity. See also specific compounds.